Process for manufacturing circuit breaker elements



United States Patent 3,407,495 PROCESS FOR MANUFACTURING CIRCUIT BREAKER ELEMENTS Leon F. Montgomery, Fullerton, Calif., assiguor to Qualtronics Corporation, Fullerton, Calif., a corporation of California No Drawing. Continuation-impart of application Ser. No. 371,754, June 1, 1964. This application May 27, 1966, Ser. No. 553,328

1 4 Claims. (Cl. 29--610) This invention relates to a method of manufacturing circuit breaker elements and, more particularly, is concerned with a device which changes from a low electrical resistance condition to a high electrical resistance condition abruptly when the current reaches a predetermined and predictable level. This application is a continuationin-part of application Ser. No. 371,754, filed June 1, 1964, now abandoned.

In Patent No. 2,923,920, there is described a device in which metallic powders are bonded in a solid mass. The device is described as an information storage device which can be triggered from a high impedance to a low impedance condition by the application of a voltage pulse. The device can be returned to a high impedance condition by passing a fairly large current through the device. Because such a device is changed from a low impedance to a high impedance condition by the passage of a large current, it operates as a switch. The fact that it is changed to a high impedance condition by a current suggests that such device might be used as a circuit breaker element. However, to operate as a circuit breaker element, the device must, among other things, be capable of going to an open circuit condition at a predetermined and stable current level. Furthermore, the device must be capable of going to an open condition at different specified current overload conditions for which the device is rated and the device must be capable of maintaining its initial rating indefinitely.

The present invention is directed to a process for manufacturing circuit elements of the type described in the above-identified patent, the process providing a method for controlling the closed circuit resistance of the element as well as the current level at which the element will go from a low resistance or closed circuit condition to a higher resistance or open circuit condition. Furthermore, the process provides a method by which the elements can be manufactured to predictable rating values and which provides a finished element that will maintain the desired rating value indefinitely.

The element is basically composed of small metallic particles which are held together by a binder or cement to provide a solid body. While various metals can be used, I have found that oxidized aluminum particles of forty-four microns or smaller in size are preferable in giving the most uniform results. For higher current ratings, particle sizes at the larger end of this size range are used, while for low current ratings, smaller particle sizes may be used. In the intermediate range, a mixture of particle sizes is used.

According to the process of the present invention, the particles of aluminum are mixed within a non-conductive binder, for example, an epoxy cement, such as Shell Epon 820. However, there are a number of suitable binder materials, such as silicon cements, rubber, Bakelite, and other high dielectric strength materials. The epoxy cement and the metal particles of aluminum are mixed in ratios by weight which vary according to the desired rating of the circuit breaker element. For example, a mixture of 40 parts by weight of epoxy is mixed with 42 parts by weight of aluminum particles of a size that will go through a 450 mesh screen to provide a circuit breaker ICC element that will trip at 20 milliamperes. One part of epoxy is mixed with 52 parts of aluminum particles of the size range of 44 microns and smaller to provide a circuit breaker element that will trip at 100 milliamperes. It has been found that by controlling the proportions by weight and to a lesser degree the size of particles, circuit breaker elements can be provided having a predetermined trip level ranging up to 600 milliamperes and higher.

When the desired mixture of the metal and epoxy is made up for a particular rating of circuit breaker element, a curing catalyst, such as Shell curing catalyst Z is added to the mixture to produce a curing and hardening of the epoxy cement. The mixture is then placed in a sealed chamber and the chamber is evacuated by a conventional mechanical forepump. The mixture is maintained at the reduced pressure for a period of up to thirty minutes. The length of time can be reduced if the mixture is slightly elevated in temperature. However, the mixture can not be heated too much or it will begin to set up. Normally the mix is maintained at room temperature during the vacuum step of the process. It has been found that the vacuum step of the process is extremely important in achieving the necessary uniformity of results required for obtaining elements which can be used as circuit breakers at predetermined current ratings.

After the evacuating step, the mix is removed from the vacuum chamber and poured into molds or encapsulated to form individual circuit breaker elements. A pair of spaced electrical leads are immersed in the encapsulated mix of each element preparatory to curing of the epoxy cement.

The most effective curing process to harden and stabilize the mix involves the process of heating the encapsulated material to a temperature of 50 C. for one hour, raising the temperature to 60 C. for another hour, and then bringing the temperature to C. for two hours. The encapsulated elements are then gradually heated over a two hour period up to 150 C. and maintained at this temperature for a twenty-four hour period, after which they are permitted to cool to room temperature.

In order to activate the circuit breaker elements, they are heated to a temperature of C. and individually pulsed across the pair of leads with a voltage pulse of several hundred volts.

Another method which has been found to give even better repeatability in operation as well as reproducibility in manufacture is to use an unsaturated polyester resin, such as, for example, Selectron 5016. The proportion by weight of the mixed metal particles is substantially the same, but the curing process is simpler. Thus the vacuum outgassing is not required and curing is performed by heating the mixture to 70 C. for an hour while in the mold, then after removing the device from the mold, slowly bringing the temperature up to C. over a two hour interval and holding this temperature for twenty-four ours.

It has further been found that the material can be conditioned electrically before curing by applying a large D.C. electrostatic field across the mix for a period of three to ten minutes after the mix material is placed in the mold and during the time it begins to gel. The mold must be of dielectric material so as not to shield the mix, and the field is preferably applied by placing electrodes on either side of the mold cavity. A DC. voltage of 15,000 to 20,000 volts is applied to the electrodes. The time is not critical but should be sufliciently long to permit the initial setting of the mix material to take place.

After the curing is complete, the device is in the o or high resistance condition but is turned on by pulsing a high voltage, e.g., 300 to 400 volts, low energy pulse across the leads.

By following the above process, it has been found that circuit breaker elements of highly predictable ratings can be manufactured. Thus by using the process of the present invention, the circuit breaker elements can be manufactured having a whole range of trip ratings merely by accurately controlling the proportion by weight of the aluminum particles to the amount of weight of binder and, to a lesser degree, controlling the size of the particles. Moreover, it has been found that by manufacturing the circuit breaker elements in the manner described above, a direct and uniform correlation is achieved between the resistance of the elements in the closed circuit or low resistance condition and the trip level of overload current at which the circuit breaker element switches to an open circuitor high resistance condition. This correlation, of course, assumes that the quantity of material and the spacing of the electrodes is maintained substantially uniform from element to element.

As circuit breaker elements, the devices manufactured according to the above-described process have the advantage that the overload current level at which they change from a low resistance to a high resistance is highly predictable and remains constant throughout the life of the element even after the element is repeatedly exposed to high overload conditions. As a circuit breaker, the devices are reset by applying a high voltage pulse across the element and, once reset, the current level at which they trip remains at the initial rated level.

What is claimed is:

1. A process for manufacturing circuit breaker elements having a predetermined current overload rating comprising the steps of mixing aluminum particles of selected particle size of the order of 44 microns and smaller together with a dielectric current and curing catalyst, the particles and cement being mixed in selected portions depending on the overload rating desired, placing the mixture in a partial vacuum for a period of time up to thirty minutes at substantially room temperature, removing the mixture and placing it in small individual molds, inserting a pair of uniformly spaced leads into the mixture in each mold, heating the mixture to an elevated temperature over an extended period of time to cure the dielectric cement, cooling to room temperature, reheating the mixture to C., and pulsing a voltage of several hundred volts across the electrodes at the reheated temperature.

2. The process as defined in claim 1 wherein the curing step includes heating the mixture to a temperature of 50 C. for an hour, then bringing the temperature gradually up over C. and holding this temperature for approximately twenty-four hours.

3. A process for manufacturing a switching device that triggers from a low resistance to a high resistance in response to a predetermined level of current passing through the device comprising the steps of mixing oxidized metal particles together with a slow curing nonconductive binder cement, placing the mixture in a nonconductive mold with a pair of spaced conductors extending into the mixture, placing the mold in a strong electrostatic field during the initial setting of the binder cement and curing the mixture to form a solid two-terminal electrical device.

4. The process as defined in claim 1 further including pulsing a voltage of several hundred volts across the conductors after the cement binder has cured.

References Cited UNITED STATES PATENTS 2,472,801 6/ 1949 Barfield et a1. 2,796,505 6/1957 Bocciarelli. 2,923,920 2/ 1960 Fitch. 2,951,817 9/1960 Myers.

WILLIAM I. BROOKS, Primary Examiner.

U.S. DEPARTMENT OF COMMERCE PATENT OFFICE Washington, D.C. 20231 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent'No. 3,407,495 October 29, 1968 Leon F. Montgomery It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3, line'33, "dielectric'current" should"read*- dielectric cement Signed and sealed this 3rd day of March 1970.

(SEAL) Attest:

Edward M. Fletcher, Jr.

Attesting Officer Commissioner of Patents WILLIAM E. SCHUYLER, JR. 

3. A PROCESS FOR MANUFACTURING A SWITCHING DEVICE THAT TRIGGERS FROM A LOW RESISTANCE TO A HIGH RESISTANCE IN RESPONSE TO A PREDETERMINED LEVEL OF CURRENT PASSING THROUGH THE DEVICE COMPRISING THE STEP OF MIXING OXIDIZED METAL PARTICLES TOGETHER WITH A SLOW CURING NONCONDUCTIVE BINDER CEMENT, PLACING THE MIXTURE IN A NONCONDUCTIVE MOLD WITH A PAIR OF SPACED CONDUCTORS EXTENDING INTO THE MIXTURE, PLACING THE MOLD IN A STRONG ELECTROSTATIC FIELD DURING THE INITIAL SETTING OF THE BINDER CEMENT AND CURING THE MIXTURE TO FORM A SOLID TWO-TERMINAL ELECTRIAL DEVICE. 